Wash, core, and slice the apples and place them in an even layer in the bottom of a 9x13 baking pan. *(If slicing the apples early, place them in a bowl and cover them with water and a couple Tbsp of lemon juice to keep them from turning brown.)
In a bowl, whisk together the white sugar, 1 Tbsp of flour, and cinnamon then sprinkle evenly over the apples.
Pour the water evenly over everything.
Crumble:
In a bowl, combine the oats, 1 cup of flour, the brown sugar, baking powder, and baking soda.
Add the melted butter and stir to combine.
Crumble evenly over the apple mixture.
Bake for about 45 minutes.
Notes:
Make Ahead: This is best served warm, but there are a couple of options for making ahead - You can completely make this ahead of time and just rewarm it in the oven on low heat before serving or you can slice the apples (cover them with water and a couple Tbsp of lemon juice to keep them from turning brown) and measure out all the ingredients so you can just throw it together closer to serving time.You can freeze the apples ahead of time, but don't freeze them with the rest of the filling and make sure they're completely thawed and very well drained before using them.Serving Size: 3x the recipe = 1 "convention" pan (9 qt). Even though this recipe says it serves 12, if making for a crowd, 1x recipe serves 16. 3x the recipe serves 48 people perfectly.